Fixed OFDM wireless MAN utilizing CPE having internal antenna

ABSTRACT

A fixed wireless access system generally comprises a consumer premise equipment (CPE) unit, that is connected via an Ethernet interface to a personal computer or local area network, and a base station unit that is connected via an Ethernet interface to a network. As such, the CPE unit is preferably easily user-installed while the base station unit is preferably tower-mounted within a 1-5 mile range of the CPE unit. Both the CPE unit and base station unit preferably incorporate an integrated data transceiver/switch that enables a radio frequency air link operating in the 2.5-2.686 GHz range. Orthogonal frequency division multiplexing is used in the uplink and downlink transmissions between CPE units and base station units.

CLAIM TO PRIORITY

The present application claims priority to the co-pending United StatesProvisional Patent Application having Application No. 60/161,107, filedOct. 22, 1999 and entitled “Fixed Wireless Access System.” Theidentified provisional application is hereby incorporated by referencein its entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of wireless datacommunication systems. More specifically, the present invention relatesto a fixed wireless metropolitan area network (MAN) that uses orthogonalfrequency division multiplexing (OFDM) carrier access modulationconfigured to allow the consumer premise equipment (CPE) to utilize anantenna deployed internally within the consumer's premise, instead ofrequiring an externally accessible antenna that has a line of sighttransmission path to a base station.

BACKGROUND OF THE INVENTION

Wireless data communication systems that utilize radio frequency (RF)signals to transmit and receive data are well known. Generally, wirelessdata communication technology has been applied to high performancelong-distance communication systems such as satellite communications ormicrowave tower telecommunications, or to short-distance local areanetwork (LAN) communication systems, such as a wireless LAN within ahome or office environment. In the case of long-distance communicationsystems, a point-to-point antenna system is required and there must be aline-of-sight transmission path between the transmitter and thereceiver. In the case of short-distance wireless LAN communication, anomni-directional antenna system can be utilized and a line-of-sighttransmission path is not required because the distances are generallyless than a mile. The reason for this difference is due to the fact thatRF signals lose power rapidly over longer distances or when transmittingthrough obstacles such as buildings or walls.

A metropolitan area network (MAN) is a network that can communicate overmedium-range distances of between about 1 to 40 miles as would betypically found in providing coverage over an entire metropolitan area.Digital subscriber loops (DSL) services are a good example of awire-based MAN system that utilizes telephone wires as the communicationmedium. Cable modem systems are another example of a wire-based MANsystem that utilizes coaxial cable as the communication medium. One ofthe primary advantages of a MAN system is that it allows for higherspeed data communications as compared to conventional telephone modemspeeds. The primary problem with such wire-based MAN systems is the costof installing and maintaining the high-quality telephone or coaxialcable communication medium. A fixed wireless MAN system has the obviousadvantage of eliminating the costs associated with installing andmaintaining a wire based communication medium.

Another advantage of a fixed wireless MAN system is that the wirelesscommunication medium can be designed to provide for higher datacommunications speeds than conventional wire-based MAN systems. Thisadvantage has caused the fixed wireless MAN systems that have beendeployed to date to be designed for ultra high performance andrelatively expensive dedicated networks. The market for these fixedwireless MAN systems has been a small number of customers who havehigh-speed data communication needs that can justify the expense andcomplicated installation of such systems on an individual basis. As aresult of the limited customer base and the need for ultra highperformance, the designs of existing fixed wireless MAN systems havedeveloped more along the lines of high performance long distancewireless communication systems.

While there are many factors to consider when designing RF communicationsystems, some of the more important factors to be considered indesigning a fixed wireless MAN system are the assigned frequency, signalmodulation and carrier access modulation. Assigned frequency refers tothe range of frequencies or oscillations of the radio signal that areavailable to be used by the system. An example is the assigned band forAM radio signals which operate between 500 KHz and 1600 KHz. Signalmodulation refers to the way in which information or data is encoded inthe RF signal. An example is the difference between amplitude modulation(AM) radio signals and frequency modulation (FM) radio signals. Carrieraccess modulation refers to the way in which the assigned carrierfrequencies are used to carry the RF signal. An example is thedifference between using a single wide channel or multiple narrowchannels over the same assigned frequency bandwidth.

For purposes of this invention, the design of a fixed wireless MANsystem is focused on frequency ranges less than 10 GHz. Othermedium-distance wireless communication systems have been developed, suchas the local multipoint distribution system (LMDS) that operate at muchhigher frequency ranges, such as 28 GHz to 31 GHz. These higherfrequencies are subject to different technical concerns and requirelarger external antenna systems that provide line-of-sight transmissionpaths from the top of one building to another.

Because of the desire for higher data speeds, all of the existing fixedwireless MAN systems have utilized more complicated schemes for signalmodulation. To support faster speed downstream transmissions, thesesystems typically use a 16-bit quadrature amplitude modulation (QAM) or64-bit QAM to transmit downstream from the base station to the CPE at adata rate of at least 10 Mbps.

Unlike the many fixed wireless LAN systems that have been developed forshort-distance communications and use a spread spectrum form of carrieraccess modulation that spreads one signal across the assigned frequencybandwidth, the relatively few fixed wireless MAN systems that have beendeveloped to date have utilized multi-carrier modulation as theircarrier access modulation. In multi-carrier modulation, the signal isdivided into several parallel data streams and these parallel datastreams are simultaneously sent along different slower speed channelsand then reassembled at the receiver to produce a higher effectivetransmission rate. The multi-carrier modulation scheme that has beendesignated by the IEEE standards committee to be used as the extensionto the 802.11 wireless LAN standard for high-speed wireless datacommunications is known as orthogonal frequency division multiplexing(OFDM). The OFDM modulation scheme makes for a more efficient use of theassigned bandwidth and improves the ability to receive higher speedtransmissions.

All of these more complicated modulation schemes for the existing fixedwireless MAN systems generally require more expensive equipment and moretransmission power at each base station. To capitalize on the increasedinvestment associated with each base station, existing fixed wirelessMAN systems have been designed to minimize the number of base stationsrequired to provide coverage for a given area. The radius of a typicalcoverage area for existing wireless man systems ranges between 10 to 30miles.

Larger coverage areas are also used to minimize the need to reuse thesame frequency channels in adjacent coverage areas. Because highertransmission powers are used to transmit at the higher data rates in allof the existing fixed wireless MAN systems, the higher power signalsprevent the reuse of the same frequency channels in adjacent coverageareas and can even preclude the reuse of the same frequency channels atdistances up to three to five times the radius of the coverage area.Consequently, larger coverage areas reduce the impact of problems causedby the inability to reuse frequencies in adjacent coverage areas.

The most significant disadvantage of larger sizes for the coverage areafor each base station is the greater potential for signal loss orattenuation between the base station and the CPE. To counteract thispotential signal loss over the larger distances and to improve receptionat the higher power, higher transmission speeds, all of the existingfixed wireless MAN systems utilize a point-to-point antenna system thatrequires a line-of-sight transmission path between the base station andan externally accessible antenna that is connected to the CPE. Forexample, see the prior art fixed wireless MAN system configuration ofFIG. 1 wherein the CPE within a single-user environment, e.g., a home,is connected to an antenna that is to the exterior of the single-userenvironment and where within a multi-user environment, e.g., a smalloffice, each CPE is connected to its own antenna that is locatedexterior to the multi-user environment.

Given the relatively limited customer base and the need for all ultrahigh-performance that has dictated the development of existing fixedwireless MAN systems, the use of externally accessible antenna thatprovide a line-of-sight transmission path is both necessary andunderstandable. It will be desirable, however, to provide for a fixedwireless MAN system that does not require the use of an externallyaccessible antenna and could be more broadly deployed to provide higherdata speeds more effectively to a larger number of consumers.

SUMMARY OF THE INVENTION

The needs described above are in large measure met by a fixed OFDMwireless MAN system of the present invention. The fixed wireless accesssystem generally comprises a consumer premise equipment (CPE) unit thatis connected via an Ethernet interface to a small office/home officepersonal computer or local area network, and a base station unit that isconnected via an Ethernet interface to a network. The CPE unit islocated in a premise for the home or small office, has an antenna thatis deployed internally within that premise and is easily user-installed.The base station unit is preferably tower-mounted within a 1-5 milerange of the CPE unit. The CPE unit preferably incorporates an internal,integrated data transceiver/switch that allows it to receive a digitalsignal from a computer or network, transform that signal to an analogformat, and transmit the analog signal via radio frequency technology,preferably operating in the 2.5-2.686 GHz range, to a base station unit.The base station unit preferably incorporates an integrated datatransceiver/switch. Upon receiving the signal, the base station unittransforms the analog signal back to a digital signal and passes thatsignal through the Ethernet connection to the personal computer, LAN,and/or network. Orthogonal frequency division multiplexing is used inthe uplink and downlink transmissions between CPE units and base stationunits.

The fixed wireless access system transmits utilizing OFDM signals thatincorporate OFDM symbols. The OFDM symbols are presented without atraining symbol and are detected in a symbol-by-symbol manner.

The fixed wireless access system utilizes a framed downlink transmissionand an unframed uplink transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides an overview of a prior art fixed wireless MAN systemutilizing external antennas.

FIG. 2 provides an overview of a fixed OFDM wireless MAN system of thepresent invention utilizing internal antennas.

FIG. 3 depicts an overview of a single sector set-up within a cell of afixed wireless access system of the present invention.

FIG. 4 depicts a cellular system of the present invention.

FIG. 5 depicts a standard, prior cellular re-use pattern.

FIG. 6 depicts a prior art cellular re-use pattern utilizing TDMA.

FIG. 7 depicts the preferred cellular re-use pattern of the presentinvention.

FIG. 8 depicts the layout of the uplink and downlink transmission slotsused with the system of the present invention as well as the layout ofthe message packets contained within the slots.

FIG. 9 depicts, in block diagram format, the processing of a bit streamof a data package that is transmitted or received by radio frequencywithin the fixed wireless access system of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An overview of a fixed OFDM wireless metropolitan area network (MAN)with computer premise equipment (CPE) utilizing internal antennas of thepresent invention is shown in FIG. 2. As depicted, the fixed wirelessaccess system 10 of the present invention may be configured for asingle-user environment or a multi-user environment, e.g., a local areanetwork. System 10 operates to transfer data from and to users of system10 through use of high-reliability radio transmission technology. System10 is especially applicable to the residential and small office/homeoffice (SOHO) markets.

Referring now to FIG. 3, an overview of a single sector set-up within acell of fixed wireless access system 10 is shown. As shown in FIG. 3,system 10 generally comprises one or more hosts, e.g., one or more hostcomputers 12 and/or one or more local area network servers 13, which areconnected to one or more customer premise equipment (CPE) units 14 viaan Ethernet connection 16. Each CPE unit 14 communicates with one ormore base station units 18 within system 10 via radio frequency. Eachbase station unit 18 is connected via an Ethernet interface 19 to one ormore of various types of networks 19, or switching fabrics, e.g.,asynchronous transfer modes (ATM).

I. System Components, Component Distribution and Component Recognition

Each CPE unit 14 incorporates hardware necessary to implement Ethernetcommunication with a user's personal computer 12 or LAN server, as wellas radio frequency communication with base station units 18. Thathardware is preferably implemented, at least in part, by use of fieldprogrammable gate array (FPGA) technology, or ASIC technology, and ispreferably designed for a maximum power consumption of approximately 10Watts. More specifically, each CPE unit 14 preferably incorporates anintegrated data transceiver/switch and one or more Ethernet connectors,e.g., 10Base-T RJ45 connector (10BASE-T is a transmission mediumspecified by IEEE 802.3 that carries information at rates up to 10 Mbpsin baseband form using twisted pair conductors, also called unshieldedtwisted pair (UTP) wire). With respect to the integrated datatransceiver/switch, it should be noted that individual components may beused without departing from the spirit or scope of the invention.

For ease of CPE unit 14 installation, the integrated datatransceiver/switch preferably incorporates an integral directionalantenna that allows CPE unit 14 to be installed by a customer near anassociated host computer 12 and within the customer's premise. The useof a standard Ethernet connector 22 further enhances the ease ofinstallation of CPE unit 14 and allows CPE unit 14 to easily beuser-installed for communication with their host computer 12 or localarea network server 13. CPE unit 14 is preferably of a size and shape sothat it may be positioned and/or mounted atop a desk, which again addsto the ease of a user installation.

Base station unit 18 incorporates hardware necessary to implementEthernet communication with one or more of various types of networks 19,or switching fabrics, e.g., asynchronous transfer modes (ATM), as wellas radio frequency communication with CPE units 14. That hardware ispreferably implemented, at least in part, by use of FPGA technology orASIC technology and is preferably designed for a maximum powerconsumption of approximately 100 Watts. More specifically, each basestation unit 18, similar to each CPE unit 14, preferably incorporates anintegrated data transceiver/switch and one or more Ethernet connectors,e.g., 10Base-T RJ45 connector. With respect to the integrated datatransceiver/switch, it should be noted that individual components may beused without departing from the spirit or scope of the invention. Basestation unit 18 is preferably additionally equipped with a globalpositioning system (GPS) receiver to provide a time reference, forsystem resolution and accuracy. A GPS time pulse is preferably used bysystem 10 to provide synchronization over the geographically distributedbase station units 18 to avoid interference between base station units18. With respect to the integrated data transceiver/switch, it should benoted that individual components may be used without departing from thespirit or scope of the invention.

As per FIG. 3, base station unit 18 is preferably tower-mounted tofacilitate an expanded, non line-of-sight communication radius. The highsystem gain provided by the transmit levels, antenna gains, and receiversensitivity allow for non line-of-sight operation of base station unit18. If base station unit 18 is mounted at the bottom of the tower, anextended length of coaxial cable is required between base station unit18 and its antenna The longer coaxial cable run produces more loss inthe system gain and will reduce the operational distance for a givenlevel of non line-of-sight coverage.

Each base station unit 18 is positioned per a distributed cellularsystem 30, see FIG. 4, wherein each cell 32 preferably includes one ormore sectors 34, and each sector 34 preferably includes one base stationunit 18. FIG. 4 is a diagram of an exemplary distributed cellular system30 wherein each cell 32 has six sectors 34. Each cell 32 preferably hasa communication radius of approximately 1 to 5 miles, with a typicalradius of 3 miles. However, the use of a cellular, sectorized basestation unit 18 deployment does not restrict the use of a singleomni-directional base station unit 18. More specifically, it is notnecessary to have a cell with multiple sectors to operate as a singlecell operation. In the instance of a small geographic area, e.g., lessthan a three mile radius, where the potential user base is low and asingle base station unit 18 could meet the data throughput capacity, asingle base station could be installed with a high gain omni-directionalantenna.

Once each CPE unit 14 and each base station unit 18 have been properlyinstalled, each is capable of transmitting and receiving communicationsignals to and/or from each other. In the most basic of terms, thecombined effect of radio frequency communication between CPE unit 14 andbase station unit 18 is that of a standard Ethernet switch, with certainadded enhancements. For example, radio frequency communication isfacilitated between units 14 and 18 due to the fact that each CPE unit14 and each base station unit 18 has been assigned a unique address,similar to an Ethernet switching system. Further, the radio frequencycommunication between units 14 and 18 preferably occurs in the form of adata packet which includes a source and/or destination addressindicating which CPE unit 14 or base station unit 18 the communicationsignal is from and/or to, respectively, which again is similar to anEthernet switching system. Broadcast traffic, e.g., traffic sent to allunits within system 10, may also be communicated between base stationunits 18 and CPE units 14, similar to an Ethernet switching system.

Thus, just as an Ethernet switch enhances the operation of an Ethernetsystem, the switching configuration provided by CPE unit 14 and basestation unit 18 operates to increase the performance of system 10 byallowing only essential traffic to travel between CPE units 14 and basestation units 18; data packets are filtered or forwarded based upontheir source and/or destination addresses without intervention byintermediate base station units 18, i.e., distributed switching.Further, like an Ethernet system, CPE unit 14 and base station unit 18preferably implement dynamic host control protocol (DHCP), a protocolthat is observed by CPE unit 14 and base station unit 18 to dynamicallydiscover the low level physical network hardware address thatcorresponds to the high level internet protocol (IP) address of hostcomputers 12 attached to a CPE unit 14.

More specifically, when a CPE unit first comes on-line, it begins tomonitor for base station unit 18 signals through use of its transceiver.When CPE unit 14 detects a base station unit 18 signal of sufficientquality, CPE unit 14 registers with base station unit 18. Base stationunit 18 uses an authentication server within network 20 to determine ifCPE unit 14 is allowed and to determine how many host computers 12 maybe attached to CPE unit 14. Base station unit 18 then either denies oracknowledges CPE unit 14 with the allowed number of host computers 12.Upon becoming registered with one of base station units 18, CPE unit 14enters a learning phase whereby CPE unit 14 operates to learn the level3 address and Ethernet physical layer address by observing traffic. Thetraffic observed is that of one of host computers 12 requesting a level3 address from a server on the data communications network, i.e., LAN13, and that of the response of the server, which is preferably in DHCP.

Upon observing traffic, CPE unit 14 creates a table of the attached hostcomputer(s) level 3, IP address and the associated Ethernet low levelphysical network hardware address. In creating this table, CPE unit 14is able to ensure that it will not transmit messages over the air linkto base station unit 18 that have a level 3 address destination thatcorresponds to a host computer 12 that is already attached to CPE unit14 via LAN 13 interface. Similar to CPE unit 14, base station unit 18operates to observe traffic and create a table of the host computer(s)level 3, IP address, the associated Ethernet low level physical networkhardware address, and the associated over-the-air hardware address ofCPE unit 14. In creating this table base station unit 18 is able toensure that that it will not transmit messages over the air link whenthe message includes a level 3 address destination that is not in theaddress table of base station unit 18.

Further, like an Ethernet system, CPE unit 14 and base station unit 18preferably implement address resolution protocol (ARP), a protocol thatis used by end devices, host computers and other computers attached tothe network, to dynamically discover the Ethernet low level physicalnetwork hardware address of an attached host computer 12 thatcorresponds to the associated IP address of the said host computer 12.

However, unlike standard Ethernet systems, fixed wireless access system10 provides for ARP proxy wherein one of base station units 18 mayanswer ARP requests intended for a host computer 12 attached to a CPEunit 14. By acting on behalf of a CPE unit 14, the intercepting basestation unit 18 accepts responsibility for the routed data packet andmay respond thereto, e.g., base station unit 18 may pass back the actualEthernet MAC address of CPE unit 14. Of course, other and/or additionalproxy protocols may be used without departing from the spirit or scopeof the invention. By using ARP and ARP proxy, channel capacity may beconserved and the efficiency of system 10 increased, i.e., broadcasttraffic over the air is reduced. Additionally, CPE unit 14 observes datatraffic of the host computer(s) 12 that are attached to CPE unit 14. Ifthe traffic is destined to another host computer 12 that is alsoattached to CPE unit 14, then CPE unit 14 does not transmit that trafficto base station unit 18, therefore channel capacity may be conserved andthe efficiency of system 10 increased.

CPE unit 14 preferably incorporates a roaming function allowing the CPEunit 14 to be moved from a premise within the range of one base stationunit 18 to a premise within the range of another, or to switch basestations 18 if one should go off the air. CPE unit 14 monitors thequality of all base station unit 18 signals and registers with adifferent base station unit 18 when the signal of the current basestation unit 18 degrades below that of another base station unit 18. Aswith the original base station unit 18, when a change occurs CPE unit 14registers with the new base station unit 18 and, additionally, passesthe level 3 address and Ethernet physical layer address table of thosehost computers 12 connected to CPE unit 14 to the new base station unit18 to enable proper synchronization of the tables between CPE unit andthe new base station unit 18. The new base station 18 then performsgratuitous ARPs to cause table updating of the former base station unit18 in order to speed the process of the base station units 18 properlyswitching traffic to CPE unit 14 for its associated host computers 12.

Moreover, a host computer 12 can be disconnected from one CPE unit 14and connected to a different CPE unit 14. The new CPE unit 14 is thenable to observe, via traffic, that another host computer 12 is active onits LAN 13 interface. The new CPE unit 14 then performs a registrationwith the added host computer 12 adding the level 3 address and Ethernetphysical layer address of the added host computer 12 to its table. Thebase station unit 18 associated with the new CPE unit 14 then recognizesthat a new host computer 12 has been added and operates to create a newentry in the base station unit address table for the new host computer12. Base station unit 18 additionally performs a gratuitous ARP toupdate other base station units 18.

II. System Data Transmission

Fixed wireless access system 10 preferably operates in the 2.5-2.686 GHzinstructional television fixed service/multipoint distribution service(ITFS/MDS) frequency range. The FCC licenses these frequencies as 31channels, each with a 6 MHz bandwidth for two-way digital communication.In a recent order, the FCC has determined that channel licensees will beissued a blanket license thereby eliminating the need for each user toregister their CPE unit 14 and eliminating the need for each basestation unit 18 to be individually registered.

As indicated above, system 10 is preferably a cellular system 30 whereineach cell 32 in the system is divided into one or more sectors 34. One 6MHz channel may be used to support a complete system by using acombination of cellular frequency reuse and a time division multiplexmethod. Alternatively, more than one 6 MHz channel may be used; addingmore channels increases system 10 capacity for radio frequencycommunication capacity and throughput.

A preferred system 10, as shown in FIG. 4, utilizes a cellular system 30wherein each cell 32 is divided into six sectors 34 and provided withsix channels such that a sector 34 may use a channel all the time. Inthis preferred configuration, system 10 provides a 1:1 reuse pattern, atransmission rate of 9 Mbps per sector (54 Mbps per cell), and a datathroughput rate of 3 Mbps per sector (18 Mbps per cell). Preferredsystem 10 is able to support approximately 300 simultaneous active usersper sector (1800 per cell) and approximately 1000-1500 subscribers persector (6000-9000 per cell). At a minimum system 10 is designed tosupport at least 250 simultaneous active users per sector.

Prior art wireless systems generally require at least one ring of cellsof separation for reuse of a frequency. For example, refer to prior artFIG. 5 wherein there are three frequencies being used within cells 32,as indicated by the three different shadings. In the configuration ofFIG. 5, the cellular system operates to separate each cell that sharesthe same channel set by at least one cell 32 in order to minimizeinterference while letting the same frequencies be used in another partof the system. In another prior art, wireless system time divisionmultiple access (TDMA) is used to diminish frequency interference amongcells. For example, refer to prior art FIG. 6 where each cell 32 isdivided into sectors 34, each sector 34 having its own frequencychannel, the channels being repeated in the next proximate cell 32. Toenable this frequency reuse, TDMA is used to give each user a uniquetime slot within the channel. As such, in the bottom cell 32, sector 1,a user transmits according to the indicated stepped time signal, in theadjacent right cell 32, a user transmits according to the indicatedstepped time signal, i.e., after the bottom cell 32 transmits, and inthe adjacent top cell 32, a user transmits according to the indicatedstepped time signal, i.e., after the adjacent right cell 32 transmits,and so on, so that each sector 1 in each cell transmits at a differenttime. However, according to the present invention through the use ofquadrature phase-shift keying (QPSK) and the decreased diameter of eachcell, described further below, neither a separation of cells 32 norinter-cell TDMA is required, see FIG. 7.

In alternative embodiments of the present invention, each cell 32 may beprovided with three sectors 34 whereby the time division multiplexmethod used within that cell is based on a two cell pattern (sixsectors). When the two cell pattern is provided with a single 6 MHzchannel, transmission occurs one-sixth of the time in each sector, whenthe two cell pattern is provided with two 6 MHz channels, transmissionoccurs one-third of the time in each sector and, when the two cellpattern is provided with three 6 Mhz channels, transmission occursone-half the time. Changing cell and sector patterns, of course, has anaffect on transmission rates, data throughput rates, and the number ofusers that may be supported by system 10. However, the ability to timeshare, e.g., 1:1, 1:2, 1:3, 1:4, 1:6, etc., allows deployment of asystem 10 with a low number of frequencies for a given area to becovered. It should be noted that other cell, sector and channelconfigurations may be used within system 10 without departing from thespirit or scope of the invention. However, it should also be noted thatincreasing the number of sectors increases the overall cost of basestation unit 18 by increasing the number of separate antennas that arethen required for each base station unit 18.

Regardless of the exact cellular layout and intra-cell time divisionmultiplex duty cycle, each sector 34 preferably uses its providedchannel for data packet transmissions for increments of times calledframes. System 10 preferably uses time division duplex (TDD) to supporttwo-way communication in each sector 34. Each frame is divided into twomain parts, a downlink transmission time and an uplink transmissiontime. The downlink transmission time preferably allows for base unit 18to transmit in one of a plurality of downlink channel slots 100, seeFIG. 8. Likewise, the uplink transmission time preferably allows for CPEunits 14 to transmit in one of a plurality of uplink channel slots 102.There is preferably a variable ratio of downlink channel slots 100 touplink channel slots 102 to allow for adaptation of system datathroughput rates of the given type of communication traffic. The ratiois a preferably a configurable parameter but may be changed duringoperation without departing from the spirit or scope of the invention.

Each downlink and uplink channel slot preferably contains thetransmission of a single OFDM signal that contains a packet of data(OFDM is preferred to digital spread spectrum as digital spread spectrumdoes not provide enough power for each symbol that is transmitted overthe entire frequency; increasing the power to support for longertransmission distances results in a splattering of the power of thesignal beyond the assigned bandwidth). The timing of total frameduration is preferably configurable to a preferred standard time length.However, the duration of each frame may vary in length from one frame tothe next and may vary between cells and sectors. Note that to providesignaling and a time/frequency reference for uplink operation, thedownlink of a given sector 34 preferably transmits for the duration ofthe downlink transmission time, even if there is no data to be sent onthe downlink for a given frame or portion of a frame.

Referring to FIG. 8, each downlink transmission preferably contains adownlink message packet 104, comprising a continuous byte stream thathas been generated by host computer 12 or network 19. Each byte streambegins and ends with a flag 106, e.g., 1 or 2 bytes, to mark thebeginning and ending of the message packet. In between flags 106, eachbyte stream preferably includes a 4 byte destination address 108, a 2byte length/type field 110, up to 2 k of data bytes 112, and a 4 bytecyclic redundancy code (CRC) 114, which covers the address field 108,the length/type field 110, and the data 112.

Additionally, the downlink transmission portion is framed using an airlink MAC protocol and preferably contains a frame header field (FH) 116and a plurality of uplink channel status fields (UCS) 118, the UCSfields 118 appearing at intervals of one downlink slot time in thedownlink transmission. In addition, each downlink OFDM symbol beginswith an eight-bit symbol sequence flag (SSF) 119, which indicates if adownlink symbol contains a frame header field 116. As such, each OFDMsymbol contains a packet of data and detection aiding informationsufficient to demodulate the symbol; distinct OFDM symbols containingknown, fixed information for training, i.e., data that is embedded inthe symbol to allow the receiver to acquire and lock on to atransmission, is not utilized.

Frame header field 116 contains the over-the-air address of base stationunit 18 and other information that is specific to the given base stationunit 18 for overall operation of base station unit and CPE unit(s) 14that are using the given base station unit 18. The preferredconfiguration of frame header field 116 provides for a total of eightbytes including: (1) several flags (1 bit each) for the start of asuper-frame, the end of a super-frame, and idle symbol; (2) systemidentifier, 4 bits; (3) transmit power level, 4 bits; (4) sector/cellbase station unit address, 4 bytes; (5) a bias number indicating thenumber of OFDM symbols in the downlink portion of the frame, 4 bits; (6)time division multiplexing re-use factor (e.g., 1:1, 1:2, 1:3, etc), 4bits; and (7) cyclic redundancy code (CRC), 1 byte.

Uplink channel status (UCS) field 118 contains information about whetheran uplink channel slot 102 is being used. As such, there is a UCS field118 in each of the first “n” downlink OFDM symbols, where “n” is thenumber of uplink slots in the frame. If slot 102 is being used, the UCS118 contains: (1) the over-the-air address of CPE unit 14 that is usingthe specific uplink channel slot 102; (2) whether uplink channel slot102 is reserved, and for which CPE unit 14; and (3) other pertinentinformation for control of the given uplink channel slot 102. Apreferred configuration of UCS field 118 provides for a total of sixbytes including: (1) mobile address, 4 bytes; (2) slot in use, 1 bit;(3) Ack, 1 bit; (4) preempt, 1 bit; (5) reserved, 2 bits; (6) Quality ofService (QoS), 3 bits; and (7) cyclic redundancy code (CRC), 1 byte.

The mobile address of UCS field 118 generally refers to the CPE unit 14that used the given slot 102 in the preceding frame. However, it mayrefer to a CPE unit 14 that will use slot 102 in the uplink transmitportion of the current/next frame but may not have used slot 102previously. “Slot in use” refers to whether the given slot 102 will beavailable for random access in the CPE unit 14 transmit portion of thecurrent frame. “Ack” refers to the results of the uplink transmission inthe given slot 102 in the preceding frame. A CPE unit 14 must retransmitany incorrect block before transmitting a new block. “Preempt” meansslot 102 is reserved for a “new” CPE unit 14 in the CPE unit transmitportion of the next frame. The “reserve” bits are not used. “Quality ofService” (QoS) refers to priority of slot 102 in the CPE unit transmitportion of the current frame, i.e., only users of specified or higherpriority will be allowed to transmit random access bursts in the givenslot 102 in the uplink transmit portion of the current frame. The CRC isthe same polynomial that is used in the frame header field 116 andcovers all of the other fields in the UCS field 118.

The downlink provides media access control (MAC) by CPE unit(s) 14 fortransmission on the uplink via UCS field 118. The MAC provided by thedownlink preferably uses airlink MAC protocol. This MAC preferably actsas a slotted-aloha media access, providing users with on demand accessto the airlink between CPE unit 14 and base station unit 18, withimplicit additional slot reservation for extended message transmissionfrom a CPE unit 14. Quality of service (QoS) is preferably provided inUCS fields 118 to control the services that are allowed access.

The byte stream is conditioned for transmission by CPE unit 14 or basestation unit 18 per the lower level of the block diagram in FIG. 9. Asshown, the byte stream is first subjected to forward error correctioncoding, as provided by a Reed/Solomon block encoder 40, and aconvolutional encoder 42. Reed/Solomon block encoder 40 operates to addbytes of Reed/Solomon parity, e.g., ten bytes of parity, to the bytestream in which a certain number of byte errors, e.g., five byte errors,can be corrected. After Reed/Solomon block encoder 40, the byte streamis applied in serial bit stream fashion to convolution encoder 42.Convolutional encoder 42 is preferably a half-rate convolutional encoderthat operates to add redundancy to the bit stream. Note the Reed/Solomoncode word is preferably input to convolutional encoder 42 with aconstraint length of 7, a depth of 35, and a code rate of 0.5. Ofcourse, other constraint lengths and code rates may be used withoutdeparting from the spirit or scope of the invention.

In the preferred embodiment, the byte stream is coded with theReed/Solomon block encoder 40 and ½-rate convolutional encoder 42 to use672 carriers. More specifically, these 672 carriers, which carry datainformation, are modulated with two bits providing 1344 bits of datathat are transmitted per symbol. These 1344 bits of data are ½-rateconvolutional encoded for random errors leaving 672 bits of data whenreceived and convolution decoded by the receiver. The 672 bits comprise84 bytes of data that are separated into 74 bytes of payload data to betransferred and 10 bytes of error correction using Reed/Solomonencoding. When the 84 bytes of data are received, Reed/Solomon decodingerror correction is performed (as described below) to correct up to fivebytes of data that may be in error, which corrects for burst errors thatare received.

The bit stream leaving convolutional encoder 42 is provided to a signalmapper 44 which is preferably comprised of interleaver block 46 and“bits to QPSK symbols” block 48. Signal mapper 44 operates to interleavethe output bits from convolutional encoder with a specific span anddepth, e.g., 32 and 42, respectively. The bit values of 1/0 are thencoded to −1/1 and unmodulated dibits, e.g., three unmodulated dibits(0,0), are then inserted at the center of the bit sequence to form atotal sequence of 675 information dibits, each of which modulates aquadrature phase-shift keying (QPSK) subsymbol. The nulling out, or notmodulating, of the center three carriers removes the need to preserve DCand low frequency content in the modulated signal, which ease the designconstraints and implementation of a transmitter and receiver.

The use of QPSK modulation on the information carriers allows for anoptimized cellular system. More specifically, the use of QPSK modulationon the carriers provides for an optimum carrier-to-interference ratiofor a given data throughput rate. This optimum carrier-to-interferenceratio allows for a cellular style of deployment that uses a 1:1frequency reuse pattern. This allows each cell to use the same sixfrequencies in a six-sectored cell. Higher orders of modulation requirea larger carrier-to-interference ratio therefore requiring more, i.e.,three times or more, frequencies than a QPSK modulated system.

To further explain, reference is made to FIG. 10 which is a diagram thatshows the interference for a 1:1 repeating pattern of a cell that hassix 60° sectors with a 30° offset, wherein the distance of 1 isreferenced to a vertex of a sector, R. In this diagram, site X is themain transmitting site. The subscribers that would be interfered withare A, B, and C. The sites that would interfere would be T and U. Thecells below and to the right of T and U would also add to theinterference but to a much lesser degree than T and U. The levels ofinterference then, are as follows $\left( {A\frac{1}{R^{4}}} \right.$propagation loss factor is used for the following analysis):

-   -   1. “A” would be interfered by T and U. The level of interference        is approximately −14.84 dB.    -   2. “B” would be interfered by T and U. The level of interference        is approximately −14.84 dB.    -   3. “C” would be interfered by T and U. The level of interference        is approximately −13.9 dB.        An additional 2 to 4 dB of protection is available when the        radiating patterns of the directional antennas are taken into        account.

The signaling of OFDM using QPSK requires only 5 dB of SNR(signal-to-noise ratio) protection to achieve a 10⁻⁶ bit error rate(BER). The six sector cell provides at least an additional 8 dB ofinterference protection. Higher order modulations require a higher SNRcompared to QPSK for the same symbol error rate. The following tableshows the level of modulation and the additional protection required forthe higher level modulations relative to QPSK. Added TransmissionProtection Modulation Bits/sec/Hz Rate Required Reuse Eff BPSK 1 2.5Mbps  0.0 dB 1:1 0.50 QPSK 2   5 Mbps  0.0 dB 1:1 1.00  16 QAM 4  10Mbps  7.0 dB 3:1 0.66  64 QAM 6  15 Mbps 13.2 dB 5:1 0.60 256 QAM 8  20Mbps 19.3 dB 7:1 0.57

The transmission rate is an example of a transmission rate forcomparison between the modulations. The added protection is theadditional amount of SNR required for the higher modulation to achievethe same symbol rate error as the QPSK. This added protection holds truefor the interference from co-channel sites. The added protection levelsthat are required are close or exceed the available margin from a sixsector 1:1 cellular pattern as described previously. The reuse factor isthe number of channel sets that are required to create a reuse patternthat is capable of providing the required protection. The rule of thumbis that for every doubling of order of modulation, there is an increaseof 3 dB needed for additional protection. This increase of 3 dB in powertranslates into an increase in propagation distance that results in theinability to achieve a one-to-one frequency reuse ratio between adjacentcells.

An efficiency factor can then be calculated as bits/sec/Hz/area relativeto the QPSK. The present invention maximizes this efficiency factor tocreate a highly efficient cellular system for a fixed OFDM wireless MAN.The present invention recognizes that the higher order modulations havea lower efficiency factor when an entire cellular network is considered.Therefore QPSK is the optimum modulation for a cellularized system thatuses a minimal amount of spectrum over a given area in a cellularnetwork. It should also be noted that the higher order modulationsrequire signal levels for higher fading margins due to multi-pathconditions.

Next, continuing with the signal conditioning discussion and referringonce again to FIG. 9, modulation, preferably orthogonal frequencydivision modulation (OFDM) 50, is performed on the QPSK subsymbolsexiting signal mapper 44. OFDM 50, as indicated in FIG. 9, preferablyincludes the following steps. First, pilot subsymbols are inserted withmodulating dibit value (1,1) evenly among the information dibits,unmodulated guard subsymbols are inserted at the top and bottom of the 6MHz channel, and out-of-band subsymbols are added to make a desiredtotal sequence length of subsymbols, e.g., 1024 subsymbols, per OFDMsymbol, see block 52. Next, a sign bit randomizer is applied to thesubsymbols, see block 54. More specifically, the sequence of subsymbolsis preferably multiplied by a pseudorandom noise (PRN) sequence toeliminate amplitude spikes due to the nonrandom nature of thedata+pilot+guard+out-of-band subsymbols.

The next step in OFDM preferably comprises performing an inversefast-Fourier transform on the now randomized subsymbol sequence, seeblock 56. After completion of the transform, a cyclic prefix/postfix isinserted at the start of the downlink symbol, see block 58. Withmodulation now complete, the digital sequence is preferably submitted toa low pass filter and, if necessary, interpolated to higher frequencyrate prior to input to a digital-to-analog converter, see block 60.Finally, the sequence is submitted to a digital-to-analog converter 62and transmitted from CPE unit 14 or base station unit 18 via analogradio circuitry.

OFDM operates, at least in part, to combat the effects, e.g.,constructive and destructive interference, and phase shifting of thesignal, of multipath. Multipath is a propagation phenomenon that resultsin radio signals reaching a receiving antenna by two or more paths.

Referring once again to FIG. 8, each uplink transmission preferablycontains an uplink message packet 120, comprising a continuous bytestream that has been generated by a computer 12 or network 19. Each bytestream preferably includes a 4 byte destination address 122, a 4 bytesource address 124, a 2 byte length/type field 126, 60 data bytes 128,and a 32 bit cyclic redundancy code (CRC) 130, which covers both addressfields 122 and 124, the length/type field 126, and the data 128. Notethat with an uplink transmission, message packet 120 is not framed, aswith the downlink transmission, however, a fixed number, e.g., six, ofuplink channel slots 102 are expected. System 10 may be configured toallow for any given CPE unit 14 to transmit in only one uplink channelslot 102 of a given frame. However, system 10 may alternatively beconfigured to enable a plurality of uplink messages from a single CPEunit 14 to be processed simultaneously, up to the number of uplink slots102 per frame. Thus, subject to control by the MAC layer, an individualCPE unit 14 can increase its uplink throughput by using two or moreuplink slots 102 in each frame if desired, up to the total number ofuplink slots 102 in the frame.

The byte stream is conditioned for reception by CPE unit 14 or basestation unit 18 per the upper level of the block diagram in FIG. 9. Asindicated, an analog signal is received by CPE unit 14 or base stationunit 18 via analog radio circuitry. The analog signal is then submittedto an analog-to-digital converter 70. The output of analog-to-digitalconverter is sampled and provided as feedback within an automatic gaincontrol loop so that the analog-to-digital converter is maintained in alinear operating range, see block 72. The output of analog-to-digitalconverter is also submitted to “digital LPF and decimator” block 74whereby the digital output is shifted into DSP preferably using fieldprogrammable gate array (FPGA) or application specific integratedcircuit (ASIC) technology, and low pass filtered. The signal is now inthe form of an OFDM symbol.

Operating on the OFDM symbol, the next step in completing reception isto remove the cyclic prefix and postfix from the OFDM symbol, see block76. A fast-Fourier transform is then performed on the received OFDMsymbol, see block 78. A sign bit de-randomizer is then implemented, seeblock 80. Coarse timing/coarse frequency and fine timing/fine frequencyof the OFDM symbol are provided by blocks 82 and 84, respectively.

Coarse timing is preferably achieved by correlating the cyclical prefixof a given OFDM symbol with the content of the symbol. Morespecifically, the cyclical prefix, which is a repetition of a portion ofthe symbol, allows the receiver to perform an auto-correlation functionto determine where the start of a symbol is in time within severalsamples. The receiver is capable of symbol-by-symbol detection once thecoarse timing has been acquired by observing several symbols (thesesymbols are not required to be fixed data content, training symbols).Coarse frequency is preferably acquired by pilot correlation. Morespecifically, the receiver performs an auto-correlation in the frequencydomain based on the pilots to determine the frequency of the receivercarrier.

Fine timing of the OFDM symbol is preferably achieved by evaluating thephase of the pilots. The pilots are transmitted at a known phase therebyallowing the receiver to use this known information to determine wherethe start of a symbol is precisely, to better than a fractional portionof a sample. Fine frequency of the OFDM symbol is preferably acquiredfrom the cyclical prefix. The cyclical prefix is used to tune thefrequency of the carrier precisely to the carrier of the transmitter.Once the receiver has acquired coarse timing and fine frequency, theneach OFDM symbol is adjusted for fine timing and coarse frequencyenabling improved symbol detection, improved sensitivity reception, andimproved error performance by the receiver.

The OFDM symbol is next submitted for demodulation which includeschannel equalization via pilot processing, see block 86. With the OFDMsignal now demodulated, the pilot, guard, and out-of-band subsymbols areextracted leaving a total sequence of information dibits, each of whichmodulated a quadrature phase-shift keying (QPSK) subsymbol, see block88. The QPSK symbols are then preferably submitted to a signal de-mapper90, which comprises block 92, wherein the QPSK symbols are returned tobit values of 1/0, and block 94, wherein the bits are de-interleaved.Signal de-mapper 90 effectively operates to place the bits in the sameorder as the originating signal to be transmitted. The output of signalde-mapper 90 is a serial bit data stream that is preferably submitted toa Viterbi decoder 96 wherein the bit rate of the serial bit data streamis reduced by one-half to correct errors. The output of the Viterbidecoder 96 is then preferably submitted to a Reed/Solomon block decoder98 which operates to correct residual errors in the submitted datastream.

The uplink data stream is then submitted to a cyclic redundancy code(CRC) check in the base station unit 18. The CRC check is a techniquefor error detection in data communications that is used to assure a datapacket has been accurately transferred. The CRC is the result of acalculation on the set of transmitted bits that the transmitter, e.g.,CPE unit 14, appended to the data packet, as described earlier withrespect to the uplink transmission. At the receiver, e.g., base stationunit 18, the calculation is repeated and the results are compared to theencoded value. The calculations are chosen to optimize error detection.If the CRC check is good, the data packet is processed. If the CRC checkis bad, then the data packet is rejected from further processing, as ifthe packet was not received at all by the base station unit 18.

In view of the above, it can be seen that fixed wireless access system10 of the present invention is able to provide multichannel multipointdistribution service (MMDS) operators maximum throughput and usercapacity per spectrum allocated with easy network deployment on both thebase station and customer sides. More specifically, system 10 cansupport a higher effective throughput, which is defined as customerdensity times data throughput rate per customer, than other existingwireless systems. With respect to the customer side, CPE unit 14 iscompletely user-installable by use of a simple Ethernet connector andrequires no registration with the FCC. Further, the cellularized andsectorized structure of the base station unit 18 design allows forcomplete frequency re-use of the allocated channel set which enablesease of network planning, and the ability to vary cell sizes consistentwith the density of subscribers, i.e., high customer density ispreferably addressed with a plurality of adjacent smaller cells 32 asopposed to a single larger cell.

With respect to a retail implementation of fixed wireless access system10 the following preferably occurs: (1) a potential end user of system10 goes to a retail electronic store to purchase CPE unit 14; (2) theend user is provided by the retailer with a contract for the serviceprovider in the area that is providing fixed wireless access system 10;(3) the end user contacts the service provider and supplies the serviceprovider with the information necessary to allow the service provider toenable the end user's specific CPE unit 14; and (4) the end userinstalls CPE unit 14 utilizing its internal antenna, as previouslydescribed, allowing interaction with system 10. The service provider isnot required to send service personnel to the end user's premise toinstall CPE unit 14. Of course, other manners of retail implementationmay be used without departing from the spirit or scope of the invention.

Applications of fixed wireless access system 10 include, but are notlimited to: (1) high-speed data applications, e.g., Internet access (DSLspeeds), remote access e-mail hosting, WAN/LAN extension, remote MISsupport services; (2) telephony, e.g., Internet telephony, voice overInternet Protocol (VoIP); and (3) video, e.g. video conferencing, videostreaming, remote video camera surveillance, distance learning,telemedicine.

The present invention may be embodied in other specific forms withoutdeparting from the spirit of the essential attributes thereof;therefore, the illustrated embodiments should be considered in allrespects as illustrative and not restrictive, reference being made tothe appended claims rather than to the foregoing description to indicatethe scope of the invention.

1. A wireless communication system, comprising: a first transceiver anda second transceiver, wherein said first and second transceiverscommunicate over a radio frequency air link permitting both uplinktransmissions and downlink transmissions, that utilize orthogonalfrequency division multiplexing (OFDM) modulation, between said firstand second transceivers, and wherein said downlink transmission isframed and said uplink transmission is unframed.
 2. The system of claim1 wherein said air link is established through a media access control(MAC) protocol.
 3. The system of claim 2 wherein said MAC protocolprovides a slotted-aloha media access with implicit reservation slotsfor a message with said transmission exceeding one slot of payloadinformation.
 4. The system of claim 1 wherein the frame of said downlinktransmission includes a field selected from a group consisting of: adestination address field, a frame header field, and a uplink channelstatus field.
 5. The system of claim 4 wherein said uplink channelstatus field includes quality of service (QoS).
 6. The system of claim 1wherein said uplink transmission is presented within a fixed number ofuplink slots.
 7. The system of claim 1 wherein said uplink transmissionsand said downlink transmissions are time division duplexed.
 8. Thesystem of claim 7 wherein said uplink transmissions are transmitted in aplurality of uplink slots and said downlink transmissions aretransmitted in a plurality of downlink slots, and wherein there is avariable ratio of said downlink slots to said uplink slots.